1. Field of the Invention
The invention disclosed herein relates to ophthalmic instruments and methods, and more particularly, relates to apparatus and methods for measuring accommodation of a lens in an eye.
2. Description of Related Art
Accommodation is the adjustment of an eye for seeing objects at different distances. Accommodation is accomplished by changes in the curvature of a lens in an eye.
Typically, accommodation is measured in a subject by asking a subject to hold a letter chart in one of his hands. The letter chart has multiple rows of letters decreasing in size from one end of the chart to the other end of the chart (i.e., top to bottom). The subject is asked to close one eye, and to focus on one of the rows of letters. The subject is then asked to bring move the letter chart closer to his eye and to stop moving the chart when the row of letters becomes blurred. The process may be repeated several times to provide an average value. The distance which the chart is moved from a focused position to a blurred position may then be used to determine the amount of accommodation of the subject""s eye. However, this method suffers from several shortcomings including that the accuracy of hand movements of the letter chart is not very accurate and repeatable, and that the size of the rows of letters increases as the letter chart moves closer to the eye. The change in size of the letters confounds the test, and results in inaccurate measurements of accommodation.
U.S. Pat. No. 4,778,268 (the ""268 patent) discloses a testing system that measures accommodation of a subject""s eye. The system is relatively complicated and awkward to use. The system includes an illumination source, a target with a small light transmitting portion and a large light blocking portion, and multiple lenses between a viewing aperture and the target. The system also uses a manually operated movement assembly to move the target and illumination source along a track. The particular configuration disclosed in the ""268 patent is necessary to create a virtual image that is viewed by the subject, and to provide the visual training capabilities of the system. The system of the ""268 patent suffers from its necessarily complex design and the inaccuracies associated with manual control of the system.
The present invention addresses the shortcomings of current techniques and systems.
An apparatus, as disclosed herein, for measuring accommodation of a lens in an eye provides consistent, controlled, and repeatable presentations of a fixed accommodative stimulus to a subject, such as a person. The apparatus determines the amplitude of accommodation based on the equivalent dioptric change of a target of the apparatus that is viewed by the subject. By making use of the Badal principle, the apparent size of the stimulus remains substantially constant at all optical distances. Thus, the subject viewing the target receives the same stimulus in terms of spatial frequencies at all distances. The constancy of the stimulus helps to improve the repeatability of the accommodative amplitude measurements.
An apparatus for measuring accommodation of a lens in an eye may have a housing; a viewing target located in the housing and located on a track that permits the target to move in the housing; a viewing aperture located in the housing to permit the subject to view the target; and a lens located between the target and the viewing aperture at a position that provides a substantially constant apparent size of the target when viewed by the subject regardless of the distance between the target and the viewing aperture.
An apparatus for measuring accommodation of a lens in an eye of a subject may also have a viewing aperture; a physical target positioned on a track so that the target can move along the track toward or away from an eye of a subject viewing the target through the viewing aperture; and a lens located in the apparatus so that when the physical target is viewed by the subject through the viewing aperture, the lens is positioned between the target and the subject""s eye, and the target has a substantially constant apparent size regardless of the distance between the target and the viewing aperture.
In addition, an apparatus for measuring accommodation of a lens in an eye of a subject may include a target located in the apparatus and moveable towards or away from an eye of a subject when the subject is viewing the target, the target being structured to maintain a substantially constant pupil size of the eye as the target is moved towards or away from the eye.
Methods of measuring accommodation of a lens in an eye of a subject include steps of moving a viewable target from a first position to a second position, monitoring the distance between the first position and the second position, and determining the accommodative amplitude based on the distance. The methods may be performed by a subject who is viewing the target, and may be performed without any assistance by another person.
Any feature or combination of features described herein are included within the scope of the present invention provided that the features included in any such combination are not mutually inconsistent as will be apparent from the context, this specification, and the knowledge of one of ordinary skill in the art.
Additional advantages and aspects of the present invention are apparent in the following detailed description.